This invention relates to purification processes and, more particularly, to a process for separating chemical species exhibiting, or capable of exhibiting, a difference in oxidation potential in solution.
At present, a number of electrochemical processes are employed to separate or purify materials, such as elemental metals and isotopes. Presently-employed electrochemical processes include electrolytic refining, electromigration, electrophoresis, electrodialysis, and other more limited techniques. In general, use of these methods is limited because of particular process requirements. Additionally, separation of some species by these methods can be so expensive due to high electrical power requirements that such separation is rendered impractical.
Electrolytic refining relies upon a difference in the oxidation potential of the species to be separated and upon electrochemical action at the electrodes to produce a desired separation. A limitation of this method is that the oxidation potentials must be substantially different to obtain significant separation.
Electromigration relies upon a difference in the migration velocity of similarly-charged species towards one of the electrodes. Fluid is flowed countercurrently to the species at a velocity intermediate that of the species to reverse the direction of one species. The disadvantage of this method is that very careful adjustments of the velocity of the counterflowing fluid must be continuously maintained and the nature of viscosity prohibits a constant and uniform flow across the cell. Furthermore, this method cannot be employed to separate species having substantially the same migration velocities and, for practical purposes, cannot be used to separate species having similar migration velocities.
Electrophoresis requires that the materials to be separated be such that they can be given charges opposite to each other and thus diverted to opposite electrodes. This method is most used in the drug industry for separating molecules that are capable of reacting with both acids and bases because their charges can be readily altered by, e.g., pH adjustment. This method cannot be employed where the materials to be separated cannot be oppositely directed by concentration or pH changes.
Electrodialysis separates by employing a physical barrier such as a membrane which preferentially passes one of the species to be separated. The disadvantage of this process is that membranes are not available to separate some species because of their similarity and, even when available, can be expensive to produce.
Other, more limited electrolytic techniques for isotope separation include preferential reduction methods used to produce heavy hydrogen by enrichment of the same over "light" hydrogen. This method requires that the reduction rate of one material be faster than that of the other material.
A uranium isotope process employing a similar sounding but quite different mechanism is described in U.S. Pat. No. 2,813,064 of A. Clark. This process employs uranous and uranyl ions which both normally flow toward the cathode. A counterflowing solution is employed to sweep the slower-flowing uranyl ions back to the anode. Because of an inherent preferential reduction and oxidation of the different uranium isotopes at the electrodes between uranous and uranyl ions, an isotope separation is produced. This process is very expensive and requires an extremely long time, e.g., 1400 hr. to effect an enrichment from 0.7111% to 0.7171%.